1. Field of the Invention
The present invention relates to an apparatus and method for monitoring an optical signal in a wavelength division multiplexing (WDM) optical transmission system, and more particularly, to an apparatus and method for accurately monitoring an optical signal in an optical transmission system irrespective of polarization mode dispersion (PMD).
2. Description of the Related Art
In a WDM optical transmission system, various optical amplifiers compensate an optical signal for losses occurring in transmission sections and at the same time amplified spontaneous emission (ASE) noise generating in the amplifiers affects the optical signal. The ASE noise lowers the optical signal-to-noise ratio (OSNR) and therefore degrades the performance of the system. Accordingly, in order to monitor and evaluate the performance of the WDM optical transmission system, the measurement of the OSNR is needed.
In a dynamically reconfigurable WDM optical transmission system, in which a plurality of wavelength channels are multiplexed and transmitted, the OSNR may differ in each channel, since optical signals transmitted from different nodes travel along different routes and pass through different numbers of amplifiers. Thus, ASE noise levels may differ in respective channels and therefore per-channel OSNR monitoring is necessary for accurate evaluation of the performance of each optical channel in WDM system.
Among the prior art to measure an OSNR, there is a technology disclosed in an article by K, Otsuka, et al, “A high-performance optical spectrum monitor with high-speed measuring time for WDM optical networks” in academic publication ‘97 European Conference on Optical Communication’. This article uses a diffraction grating and an array of optical detectors to split each wavelength spatially and measure optical power, and by doing so, measures an OSNR. The drawback of this technology is that if power of an optical signal in each signal is assumed to be constant, then even when the ASE noise levels in respective channels in dynamically reconfigurable WDM networks are different, an identical OSNR value is measured in each channel. In addition, the technology is sensitive to optical spatial alignment.
Another prior art technology to measure an OSNR is one disclosed in an article by K. Asahi, et al, “Optical performance monitor built into EDFA repeaters for WDM networks”, announced in ‘98 Optical Fiber Conference’. This technology uses an acoustic optical variable filter to scan respective wavelengths and measure optical signal power and ASE power, and by doing so, calculates the OSNR. However, this technology has a drawback that it cannot accurately measure OSNRs different in each channel.
Another prior art technology to measure an OSNR is one disclosed in U.S. Pat. No. 6,433,864 “Apparatus for monitoring optical signal-to-noise ratio of optical signals in WDM optical transmission system” filed by Y. C. Chung, et al. This technology suggests a method calculating an OSNR from beat noise detected by using electric filters and the power of an optical signal measured from a demultiplexed optical signal. This method enables to measure OSNRs different in respective channels but may cause an error due to the effect of undesired signal that may be generated at a predetermined frequency where beat noise is measured. In addition, the effect of polarization mode dispersion (PMD) may cause an error.
Another prior technology is one disclosed in an article, “OSNR monitoring technique using polarization-nulling method”, by J. H. Lee, et al., in academic publication, IEEE Photonics Technology Letters, vol. 13, 2001. This method uses a quarter wave plate and a linear polarizer to adjust polarization of an optical signal and calculates an OSNR from the output power measured when the polarizer and the signal polarization are horizontal, and the output power measured when they are vertical, that is, a signal polarization disappeared. However, when PMD exists in transmission optical fiber, an optical signal has two orthogonal polarization components delayed in time from each other, that is, two principal states of polarization. Accordingly, even though a signal polarization is adjusted, the signal polarization is not nullified by the polarizer. Therefore, this technology has a drawback that as PMD increases, an error in a measured OSNR value increases.